1. Field of the Invention
This invention relates to drying solid materials, and more particularly, to squeezing a volume of wet, solid materials to reduce the liquid content and volume of the solid materials, and to moving those materials after reducing the liquid content and volume of the solid materials; which solid materials may be non-degradable screenings previously removed from liquid flowing into a water or waste water or liquid industrial waste treatment facility.
2. Discussion of Prior Driers
Basins are used to remove certain undesired solid materials from liquid, to clarify the liquid. Basins are used, for example, in water and waste water treatment plants, and in industrial waste treatment plants. In water treatment, water drawn from a water supply has various undesired solid materials therein. One type of undesired material is non-settleable colloidal solid material. When mixed with chemicals, the non-settleable colloidal solid materials and the chemicals agglomerate to form solid materials which will settle from the water. In water and waste water treatment, the undesired solid materials include organic solids. In industrial processes, the undesired solid materials may include a variety of materials, such as fibers, which are not amenable to agglomeration and settling, for example.
Water, waste water, and liquid industrial wastes are treated in basins to remove such undesired solid materials, thereby making the water clear and suitable for use, reuse, or for further treatment, such as tertiary treatment. The word "liquid" as used below to describe the present invention refers to water, waste water and liquid industrial wastes.
These undesired solid materials include particles that are suspended in the liquid. Devices for removing the suspended materials are well-known. Such devices substantially reduce the flow rate of the liquid, and the particles therein, as in a very low flow, or quiescent, zone (or flow channel) in the basin. The suspended, low flow rate particles are subjected to the force of gravity and settle to the bottom of the basin as sludge.
However, the undesired solid materials to be removed from such liquid include items that are generally much larger than such suspended, settleable materials, and that are not removable by settling. Many of these larger materials are not regulated by government waste management authorities. These larger materials may therefore be disposed of in land fills, and include, for example, (i) man-made materials such as plastic shapes for holding six cans of soda in a six-pack; (ii) large materials such as other plastic waste, containers, construction debris, and refuse carried by, but generally not suspended in, liquid; and (iii) many other materials (such as paper towels) which are carried by the liquid, but which generally do not degrade in the manner in which bio-degradable materials (such as food) degrade.
To differentiate the undesired, suspended, settleable solid materials from such other undesired, non-settleable larger materials, these undesired, suspended, settleable solid materials are referred to herein as "settleable solids", which are removed by "settlers". The undesired items that are generally much larger than such settleable solids, and that are not removable by settling, are referred to herein as "screenable solid materials", or "screenings", because they are desirably removed by a "screen" that performs a screening process before the liquid and the settleable solids flow into the basin. The screenings (or screenable solid materials) are the materials processed according to the principles of the present invention.
If such screenings are not removed before the liquid and the settleable solids flow into the basin, (a) such screenings may interfere with the settling process during which the settleable solids are encouraged to settle to the bottom of the basin and form sludge, (b) such screenings in the sludge may interfere with removal of the sludge from the basin, and (c) the sludge collected from the bottom of the basin will include some of the screenings. Such interference with the settling process is undesirable because less settleable solids are removed per gallon of liquid processed through the settler. Also, the sludge is temporarily stored ("inventoried") before use, such that any excess (i.e., non-settleable solid) material in the inventoried sludge increases the land area, or size of other facilities, needed to store the sludge. Further, because such screenings (e.g., plastics) are not suitable for use in the manner in which the sludge is used (e.g., fertilizer for agricultural purposes), the commercial value of sludge which contains screenings is greatly reduced, increasing the net cost of the liquid clarifying operation.
Accordingly, efforts have been made to provide ways of removing screenings from the liquid and from the settleable solids before the liquid and the settleable solids flow into the basin. Applicants have previously provided improved facilities for removing the screenings from the liquid and the settleable solids before the liquid and the settleable solids flow into the basin. For example, such facilities have provided (a) a simple, lower-cost, automatic, programmed fixed bar screen cleaner; (b) a bar screen cleaner which is universally usable as either a front rake-type or a back rake-type, and which is readily convertible at a basin of a plant for either type of operation; and (c) a bar screen cleaner which is simultaneously usable for both types of raking. These bar screen cleaners are applicable, for example, to basin supply channels, or channels, constructed on low budgets, such as for small plants in which choppers are currently used, as well as for large plants. Bar screen cleaners are referred to herein as "screens", and generally operate to remove the screenings from the bar screen by reciprocating across bars of the screen. This reciprocation tends to remove the screenings from the screen in "batches" (i.e., with the screenings gathered in one group, followed by more screenings in a later separate group, rather than removing the screenings continuously.
The screenings removed from the liquid and from the settleable solids are generally wet with the liquid and are loose, having a low density (a relatively large volume for the weight of the screenings). The liquid that wets the screenings may be absorbed by the materials which compose the screenings, or, for example, the liquid may be on the surface of such materials which compose the screenings. To remove some of the liquid with which the screenings are wet, and to reduce the volume of the screenings, others have used hydraulic drives to move a piston in a cylinder. However, the hydraulic drives generally take up significant amounts of room, which may not be available adjacent to a screen. Also, the screenings are generally fed to the cylinder via an open receptacle positioned axially spaced from the cylinder. An initial portion of the stroke of the piston is used to force the screenings from the receptacle into the cylinder. As a result, the initial portion of the stroke does not immediately compress the debris. Thus, the initial portion of the stroke may be referred to as "lost motion" which increases the time required for the compressing operation, for example. Also, a batch of the screenings from the receptacle usually remains in the cylinder as a compressed batch of screenings, and remains in the cylinder with one or more previously compressed batches of screenings. Because the piston must move and compress not only a new batch of screenings, but the prior batches of screenings which remain in the cylinder, more power is needed to move the multiple batches of compressed screenings out of the cylinder.
Others have used augers which compress and move such screenings along a tube which contains the auger. However, the force that the auger can apply to the screenings is less than that of the piston, such that less compression of the screenings takes place. Also, the auger moves the compressed screenings within a tube to a discharge point, such that successive batches of the screenings remain in the tube until they are advanced to the end of the tube.
In both the piston/cylinder and the auger examples, problems arise because of the time during which the compressed screenings remain in the cylinder or tube. For example, the amount of screenings flowing in the channel to the screen may vary widely. As a result, the volume of loose, wet screenings to be processed also may vary widely. If some screenings have been forced into the auger tube or into the piston cylinder, and then there is a substantial decrease in the amount of screenings to be processed by the screen, the screenings already in either such cylinders or tubes will stay there awaiting the next batch, or batches, of screenings to be compressed. Those next batches are necessary for pushing the initial batch(es) out of the cylinder or tube. As a result of the many batches in the cylinders or tubes, odors are emitted from the compressed batches of screenings during the time period in which these batches remain in the cylinder or tube awaiting the next batch or batches from the screen. Also, flammable gases (e.g., methane) emitted from the compressed screenings increase the risk of fire in the cylinders or tubes.
An additional problem of significance is that if the screenings from the auger or piston are to be acceptable for disposal in a landfill, they must be in the form of "dry waste". Dry waste does not exhibit "free water". However, in the auger case, for example, the force that may be applied to the screenings may be insufficient to dry the screenings so that they do not exhibit free water. A test for free water, or dryness, that is used to determine whether screenings have been dried enough for disposal in a landfill is a paint filter test in which material is placed in a standard conical filter which is used to filter paint (the "Filter Test"). The requirement for lack of free water in the screenings is that the screenings are dry enough for landfill disposal only if no liquid drips from such a paint filter after the screenings have been in the paint filter for a specified number of minutes. For example, reference may be made to Method 9095, SW-846, of the Environmental Protection Agency (USEPA 1991h). In the context of the present invention, screenings which pass the Filter Test are said to be "dry", and the process of rendering the screenings "dry" (i.e., without free water) so as to pass such Filter Test is referred to herein as "drying" or "dewatering". On the other hand, screenings which would not (or do not) pass the Filter Test, are said to be "wet". The efficiency of drying is defined as a percent based on the water in a batch of the screenings with respect to the total weight of the non-settleable materials and the water in such batch.